Digital tachometers typically include some type of incremental position encoder which divides the rotational cycle of a coupled rotating shaft into a number of substantially equal and regular incremental rotations. The encoder is used to generate a high frequency pulsed signal (hereinafter referred to as a "high output signal") outputted by the tachometer. The shaft rotational (once around) speed in revolutions per second ("rps") is equal to the frequency of the high output signal in hertz divided by the number of incremental positions into which the shaft rotation is divided by the encoder.
Tachometers are often used with motors for speed control. For example, motor speed can be controlled by phase comparison of the pulses of the high output signal of the tachometer (or a signal derived from it) with the pulses from a high frequency quartz clock or other accurate timing device. When the tachometer signal and clock signal are not of the same phase or frequency, an error signal is generated having a magnitude related to the magnitude of phase difference and a polarity indicating lag or lead. The error signal is passed through a control loop filter and applied to a gain control unit for adjusting the magnitude of the current supplied to the motor. Motor speed is thus ultimately controlled by the clock.
To some degree, the high frequency output of each digital tachometer contains FM sidebands which arise from manufacturing imperfections. The sideband components caused by the tachometer are referred to as "tach runout". Sidebands produced by the motor coupled with the tach are referred to as "torque ripple". Hereinafter they will be referred to together generically as "coherent jitter". Typically, the dominant FM sideband of a tachometer output signal is displaced from the signal frequency by the rotational speed of the tachometer. Other sidebands appear at multiples of the rotational speed. All such sidebands are undesirable as they constitute false indications of velocity variation and thus limit the short term stability of the motor speed control.
One method to reduce the tach runout component has been to provide a notch filter at the once around frequency or rotational speed (rps) of the tachometer. However, this approach limits the loop bandwidth by decreasing the phase margin, especially near the once around frequency of the tachometer. Also, as the filter notch frequency is fixed and must be selected for the rotational frequency of the motor and tachometer, the filter must be replaced or a number of filters provided in order to operate the motor at more than one speed.
Another technique previously used to cancel tach runout is to use two tachometers and configure them in such a way as to have their errors cancel each other. This approach adds mechanical complexity, and the cancellation is only as good as the match between the two tachometers.